(1) Field of the Invention
The present invention relates to a solid electrolytic capacitor consisting of a structural body comprising a metal having a valve action, an oxide film formed on the surface of the metal, a semiconductor layer composed of a mixture of lead dioxide and lead sulfate, which is formed on the oxide film, and a carbon layer and/or a metal layer formed on the semiconductor layer, and a process for the preparation thereof.
(2) Description of the Related Art
A solid electrolytic capacitor using lead dioxide as the semiconductor layer is known, as disclosed in Japanese Examined Patent Publication No. 58-21,414. Since the semiconductor layer of lead dioxide in this conventional solid electrolytic capacitor is formed by thermal decomposition of a lead ion-containing reaction mother liquid on the oxide film, the oxide film is thermally cracked or is chemically damaged by a gas generated by the thermal decomposition. Therefore, when a voltage is applied to this defective solid electrolytic capacitor, an electric current is concentrated on a defective portion of the oxide film, and there is a risk of insulation breakdown. Accordingly, in order to increase the reliability of the voltage resistance, the formation voltage should be 3 to 5 times the rated voltage, and a large anode body having a large surface area should be used so as to obtain a predetermined capacitance.
As means for overcoming these defects, there is known a method in which manganese nitrate is thermally decomposed to form a layer of manganese dioxide on an oxide film layer, the manganese dioxide layer is immersed in a liquid containing lead ions and persulfate ions at very low concentrations, and a layer of lead dioxide is formed on the manganese dioxide layer by chemical deposition, as disclosed, for example, in Japanese Unexamined Patent Publication No. 54-12,447. However, this method has a problem such that, since a thermal reaction is carried out for formation of the manganese dioxide layer, thermal cracking of the oxide film or chemical damage by a generated gas cannot be avoided.
Furthermore, there is known a method in which lead dioxide is chemically deposited on an oxide film layer, as disclosed in Japanese Examined Patent Publication No. 49-29,374. However, this method has a problem such that, since a silver ion is necessary as a catalyst for the chemical deposition of lead dioxide, when this method is applied to the production of a solid electrolytic capacitor as intended in the present invention, silver or a silver compound adheres to the dielectric oxide film and the insulation resistance is reduced.
As means for eliminating the foregoing defects, there is known a method in which a tetracyanoquinodimethane salt, which is an organic semiconductor, is used as the solid electrolyte (see, for example, Japanese Unexamined Patent Publication No. 57-173,928). However, the tetracyanoquinodimethane salt is very expensive and the salt is easily affected by moisture.